This invention relates to the reduction of the amount of an organic contaminant as measured by the xe2x80x9ctotal organic carbonxe2x80x9d (TOC), in metal plating baths, and in particular to an electrochemical cell and process for selectively effecting such reduction by electrochemical oxidation.
Most electroplating baths contain additives that are used to produce uniform and bright deposits of desired properties at economically desired rates. During plating, additives are both consumed and degraded, with the degradation products building up in the plating bath. As well, other organic contaminants are introduced into the plating baths from the surrounding environment (e.g. dust, plant spores) and from the parts being plated (e.g. photoresist film leaching encountered in printed circuit board plating). When these contaminants build up to high levels they begin to interfere with the proper action of the plating additives, resulting in poor quality deposits (typically a loss of ductility, adhesion or uniformity). This invention relates to a process for removing these problem, contaminating, compounds thus allowing continued use of the plating bath solution while maintaining good quality plating.
The problem contaminants are typically removed by adsorption on activated carbon, oxidation by hydrogen peroxide at elevated temperature, oxidation by ozone, or oxidation followed by adsorption. (The activated carbon can either be used in a cartridge form or by the addition of loose granulated activated carbon followed by solution filtration). A simpler process is desirable to avoid the handling of chemicals and/or the disposal costs for the activated carbon adsorbent. After use the activated carbon retains some plating solution and thus is contaminated by a metal ion and typically a strong acid, and for these reasons is classed as a hazardous waste. Thus an ideal process would not require the addition of strong chemicals nor would it produce any waste streams.
The invention involves an electrochemical process to break down the problem organic compounds in plating mixtures, by oxidation at the anode of an electrochemical cell. By oxidation it is meant that the problem compounds are eventually mineralised, which means that the carbon based organic contaminants are transformed into benign compounds such as water molecules, carbon dioxide, and in some cases sulphate and/or nitrate. The process is designed in such a way as to efficiency destroy the problem compounds, and to minimise other possible side reactions which might alter the properties of the plating solution. By meeting these objectives the described process can produce a product which can be recycled to the plating operations with minimal processing steps. It will be appreciated that the invention covers mainly solutions of low pH, but can be extended to other plating processes.
According to one aspect of the invention, an electrochemical cell is provided, comprising
an anode compartment,
an anode located in the anode compartment,
an anolyte comprising an aqueous acidic solution containing an organic contaminant,
a pump for circulating the anolyte through the anode compartment,
a cathode compartment,
a cathode located in the cathode compartment,
a catholyte,
a pump for circulating the catholyte through the cathode compartment,
a separator between the anode and the cathode, comprising an ionically conducting solid polymer membrane material, and
a power supply for supplying dc current to the cell, wherein the anode includes an active material which is stable at acid pH and at high electrical potential, and which selectively breaks down organic compounds by electrochemical oxidation, and wherein the separator preferentially allows the passage of protons over other ions.
According to another aspect of the invention, a process is provided for selectively electrochemically reducing the amount of an organic contaminant in an aqueous solution containing such contaminant, comprising providing an electrochemical cell, the electrochemical cell comprising
an anode,
a cathode,
an ionically conductive separator between the anode and the cathode,
a catholyte, and
a dc power supply,
wherein the anode includes an active material which is stable at acid pH and at high electrical potential and which selectively breaks down organic compounds by electrochemical oxidation, and wherein the separator preferentially allows the passage of protons over other ions, the process comprising
(a) providing an electrochemical cell, the electrochemical cell comprising
an anode compartment,
an anode located in the anode compartment,
an anolyte comprising an aqueous acidic liquid mixture containing an organic contaminant,
a first pump for circulating the anolyte through the anode compartment,
a cathode compartment,
a cathode located in the cathode compartment,
a catholyte,
a second pump for circulating the catholyte through the cathode compartment,
a separator between the anode and the cathode, comprising an ionically conducting solid polymer membrane material, and
a power supply for supplying dc current to the cell,
wherein the anode includes an active material which is stable at acid pH and at high electrical potential, and which selectively breaks down organic compounds by electrochemical oxidation, and wherein the separator preferentially allows the passage of protons over other ions,
(b) activating the dc power supply to provide electrical current to the cell,
(c) activating the first pump to circulate the anolyte through the anode compartment to expose the liquid mixture to the anode to selectively break down the organic contaminant by electrochemical oxidation.
(d) activating the second pump to circulate the catholyte through the cathode compartment, and
(e) removing the aqueous solution having a reduced amount organic contaminant from the anode compartment.